Search for extraterrestrial intelligence

The search for intelligent life in the universe is perhaps the most profound of human endeavors. If extraterrestrial intelligence were discovered and communication established, it would irrevocably alter the conception of humanity’s place in the universe. Contemporary science suggests that given a suitable planetary environment and sufficient time, life will evolve, and since intelligence and technology have high survival value, they will inevitably follow. Given that there are myriad of suitable planets in the galaxy, intelligent life forms, willing and able to communicate, should be abundant.

Overview

The search for extraterrestrial intelligence is a relatively recent exploratory science that assumes that, if intelligent life more technologically advanced than us exists elsewhere in the universe, they will attempt to communicate by broadcasting messages using radio waves. Detecting and decoding such messages, however, requires considerable effort. If there is only a vanishingly small probability that an alien civilization has evolved, it would be a futile waste of time and money to search for a message. To arrive at an estimate of the number of technological civilizations with the ability and desire to communicate, Dr. Frank Drake convened a small group of scientific experts from various fields at the National Radio Astronomy Observatory (Green Bank, West Virginia) in 1961. This group formulated a simple equation to estimate the possible number of communicative civilizations. The equation is:

N = R* fp ne fl fi fc L

where N is the number of advanced technological civilizations, R represents the rate of formation of suitable stars, f_p stands for the fraction of stars with planetary systems, n_e symbolizes the number of planets in a solar system that are suitable for life, f_l corresponds to the fraction of suitable planets on which life evolves, f_i signifies the fraction of inhabited planets on which intelligent life evolves, f_c characterizes the fraction of planets with intelligence on which a technological civilization can emerge, and L embodies the length of time the technological civilization would broadcast signals. When these factors are multiplied, the equation will yield an estimate of N.

After considerable discussion, the group narrowed the possible range of values for each factor to the following values. The average rate of star formation is the 100 billion stars in our galaxy divided by its ten billion-year-age, yielding about ten stars per year. The fraction of stars with planetary systems was estimated to be about one, but the number of habitable planets was thought to be only about one in ten (0.1). Since it is believed that life evolves wherever the conditions are conducive, the fraction of habitable planets on which life evolves was taken to be one. Because intelligence has a high survival value, it was assumed that the fraction of life-bearing planets on which intelligence evolves is also one. The fraction of planets on which intelligent life develops technology is taken to be one because of technology’s high survival value and from the fact that tool-using cultures arose independently at multiple locations on Earth.

The final factor, the average lifetime of a technological civilization, is the most difficult to estimate. The Earth has had the ability to communicate by radio waves for less than a hundred years, and this state may not last another hundred years if the environmental and social problems that plague Earth are not addressed and solved. If an advanced civilization can continue in its communicative state for at least a thousand years, it could probably last for a million years. When the foregoing factors, momentarily excluding L, are multiplied, the result is a value of about one. Thus, if a technological civilization exists for only 200 years, the number of communicators in our galaxy would be only about 200 out of the 100 billion stars present; they would be few and far between. If, on the other hand, advanced civilizations can exist for at least a million years, there will be enough potential communicators to make the search worthwhile. Scientists working on the Search for Extra-Terrestrial Intelligence (SETI) project have chosen the optimistic view.

The SETI Institute (Mountain View, California) was founded in 1984 as a private, nonprofit organization dedicated to searching the skies for technological indicators of extraterrestrial intelligence. Because of the vast distance between stars, the fact that no material object can travel faster than the speed of light, and the prohibitive cost of interstellar space travel, it is assumed that an alien civilization would attempt to locate other intelligent species by using radio waves to carry nonrandom messages. Two problems then present themselves. Which stars are likely candidates for intelligent life to evolve, and what radio frequencies should be scanned for possible messages? The stars most likely to have habitable zones where planets would be neither too hot nor too cold for life are main sequence G- and K-type stars. G-type stars, such as our Sun, have a surface temperature of about 6,400 kelvins and a habitable zone that extends from Venus through Mars. K-type stars have a surface temperature of approximately 4,250 kelvins and a somewhat narrower habitable zone. Stars hotter than G probably evolve too quickly for life to develop intelligence, while stars cooler than K would have such a narrow habitable zone that a planet is unlikely to occupy it.

Only certain radio frequencies are useful for communication. From Earth’s surface, the available window of frequencies ranges from 1 gigahertz (GHz) to about 30 GHz. Frequencies lower than 1 GHz would be lost in the background radio noise of the galaxy; frequencies higher than 30 GHz are absorbed by Earth’s atmosphere. There are still far too many possible frequencies available in this window to make a search practical. There are, however, two important frequencies within this window that would be likely choices for an advanced civilization wishing to establish contact with an alien civilization to broadcast a message. These are the 14.3 GHz frequency, radiated by neutral hydrogen (H), and the 16.7 GHz frequency, emitted by the hydroxyl ion (OH). Because hydrogen is the most abundant element in the universe and because H and OH combine to form water (HOH or H₂O), any other intelligent species based on water would be reasonably likely to choose one of these frequencies on which to broadcast. Although it is possible that an alien life form could be formed from other molecules, there are valid chemical reasons to assume that this is unlikely.

Knowledge Gained

No direct evidence of extraterrestrial life has ever been observed, but there exists some significant data related to the origin of life. Complex carbon molecules, the precursors of life, have been found in interstellar clouds and in certain meteorites from our solar system. Based on the not-unreasonable assumption that intelligent life is abundant in our galaxy, the National Aeronautics and Space Administration (NASA) funded a search for alien radio messages during the late 1980s. This project was canceled in the early 1990s because politicians feared public ridicule for funding a search for “little green men.” Although the annual cost was small (about the same as one Air Force attack helicopter), it was felt that the money would be better spent elsewhere. The SETI Institute continued the search using the radio telescopes at major radio observatories. Stars deemed likely to have planets on which intelligent life could evolve are monitored for nonrandom radio emissions; none has been identified. In 2020, scientists in the Netherlands announced they believed they had discovered radio transmissions coming from remote stars known as exoplanets; however, more research was needed. In 2024, scientists from Penn State and the SETI Institute conducted the longest-ever single target search of the TRAPPIST-1 star system using the Allen Telescope Array, which was upgraded in 2022. In 2025, scientists detected radio signals from the Tau Bootis system, 50 light-years away.

A joint effort between the SETI Institute and the Radio Astronomy Laboratory at the University of California, Berkeley, is the Allen Telescope Array (ATA), a composite of 350 separate radio dish antennas at one location in northeast California. Designed for innovative astronomical research, the array is also being used to search for nonrandom extraterrestrial signals. Because the ATA is equivalent to one huge radio telescope, it can collect enormous amounts of data every hour of the day, every day of the year. It can also scan a wider portion of the sky and do so more quickly than existing telescopes. The ATA is programmed to survey one million stars within 1,000 light-years for extraterrestrial signals in the frequency range of 1 to 10 GHz. It will also survey the billions of stars of the inner galactic plane in the frequency range from 14.3 GHz to 16.7 GHz.

A study released in 2026 by researchers at the SETI Institute indicates that activity in space around stars may make radio signals from extraterrestrial civilizations more difficult to detect. Disturbances such as energy from stars and unstable charged particles near a planet could spread out what would normally be a very narrow signal, distributing it across a wider range of frequencies and making it harder to identify using traditional narrowband search methods.

Another innovative experiment searches the sky for intelligent signals in the form of powerful pulsating flashes of laser light beamed from solar systems many light-years distant. The pulse-detection system, coupled with the Lick Observatory’s 40-inch telescope, is able to detect beacons pulsating at less than a billionth of a second with an error rate of only about one per year. This system can be automated and promises results that are less ambiguous than previous optical systems in which false alarms occurred daily.

Artificial intelligence can play a valuable role in these searches. Advanced computer methods are able to analyze vast datasets and identify patterns that may suggest a deliberately created signal. In addition, AI-based methods may reduce bias compared to human researchers, who often focus on specific signal types they already consider likely or have predefined.

Context

From the ancient Greek concept that Earth was the center of the universe, humans have always assumed their place in the cosmos to be pivotal and unique. As science advanced in the sixteenth and seventeenth centuries, Nicolaus Copernicus and Johannes Kepler showed that the Sun was the center of the known universe and Earth was merely another planet. In the early twentieth century, Edwin Hubble discovered that the Sun is a very common G-type star in a galaxy of a hundred billion stars and that the Milky Way galaxy is only one among billions in the universe.

There is nothing particularly unique about the Sun. Many humans, however, clung to the notion that few other stars were likely to have Earth-like planets. By the mid-twentieth century, however, astronomers had concluded that such planetary systems would be quite common. To preserve the illusion of uniqueness, it was then assumed that life on Earth was unique. Discoveries have shown, however, that the molecular precursors of life, such as amino acids, have formed elsewhere in the solar system, leading biologists to conclude that life is probably abundant in the universe. Since intelligence has a high survival value, it follows that intelligent life would not be uncommon either. Because a civilization would necessarily be more advanced to send an intense signal through interstellar space, humans would also be denied their unique rank as the supreme intelligence. Confirmation of a superior extraterrestrial intelligence would be the kiss of death to the lingering remnants of the unjustifiably self-centered sense of our uniqueness and importance in the universe.

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